1. Field of the Invention
This invention relates generally to marine seismic surveying and more particularly to a method of determining the position of sources and receivers used in the seismic spread in marine geophysical surveying.
2. Background of the Invention
In marine seismic exploration, one or more streamer cables, each typically between 2000 and 5000 meters long and one or more acoustic pulse sources, usually air gun subarrays containing several individual air guns are towed behind a vessel in a body of water. Each streamer cable contains several sensors, typically hydrophones, spaced along the length of the streamer cable. During operation, the air guns are activated every few seconds to produce a shock wave or an acoustic pulse. The acoustic pulse is transmitted to the earth's substrata lying underneath the air gun. The acoustic pulse is reflected and refracted by the substrata layers back to the earth's surface and then to the sensors located in the streamer cables. These sensors detect the returning acoustic pulses and produce signals (dam) representative of such returning acoustic pulses. The dam is then processed to determine the structure of the earth below the surveyed area.
During the survey, the vessel is constantly moving along a predetermined course at a predetermined speed. Thus, the air guns and the sensors contained in the streamer cables are constantly moving while the survey is being performed. In order to accurately process the signals from the hydrophones (the dam), the location of the hydrophones and the location of the air gun subarrays must be determined at the time the air guns are activated.
To determine the positions of the sources and the receivers, it is typical to use a network containing a large number of different types of navigational devices. These navigational devices are placed at known locations along the streamer cable, on the air gun subarrays, on the vessel and at various other locations on various other equipment used for performing geophysical surveying. The placement of the navigational devices form a complex network which provides many hundreds of observations.
In recent years, more and more surveys are being done to obtain three-dimensional (3-D) mapping of the earth's substrata. Additionally, larger vessels using several streamer cables and air gun subarrays and multiple vessels are now routinely used for performing geophysical surveys. Use of such surveying methods requires more accurately determining the positions of the sources and receivers than has been done in the past.
To obtain more accurate positions of the sources and receivers, the trend in the industry has been to use an increasingly complex network of stations consequently increasing the number of observations by several folds.
Experience has shown that errant measurements (observations) are quite common and that if not corrected, can reduce the quality of the results obtained by processing such data. Various methods have been employed in the prior art to process the observations in real time on-board the seismic vessel. However, due to the increased complexity of the networks used, the diversity and sheer number of observations, these prior art techniques do not provide the desired accuracy, partially due to their inability to correct the errant measurements.
The present invention provides an on-line, real-time method for processing navigational observations for computing more accurate locations of the source and receiver points.